Biology Reference
In-Depth Information
organophosphorous insecticides evolved in the Montpellier region of France from 1972 to
1984, by the substitution of a costly allele in terms of physiological performance by a less
costly one (Guillemaud et al. 1998). The different molecular mechanisms of resistance to
toxins (antibiotics, herbicides, insecticides) showed differential physiological costs (Taylor
and Feyereisen 1996), and a major question was not elucidated: what is the validity of the
cost estimated in the laboratory, relatively to the real cost in the field?
In natural populations, a smaller number of studies have explored the cost of resis-
tance to chemical stress that has remained very difficult to estimate because numerous
confounding factors, not linked to toxins, can dramatically modify estimations of this
physiological cost; furthermore, we must consider that the heritability of numerous bio-
logical traits decreases from experimental conditions to natural environments (Woods and
Hoffman 2000).
Several studies involving successive generations of fish selected in the laboratory for
their resistance to contamination have characterized the cost of resistance, showing:
• Reduced size of offspring, lower fecundity, increased age at irst maturity, for
cadmium-exposed fish versus control fish in experimental exposures (Xie and
Klerks 2004)
• Higher susceptibility of ish exposed to contaminated estuarine sediments com-
pared to control fish when faced with other stressors such as UV radiation and
hypoxia (Meyer et al. 2003)
In the case of the European flounder (
Platichthys flesus),
), resistant genotypes character-
ized by their high capacity to maintain their DNA integrity in polluted estuaries, showed
lower fecundity and condition index, compared to other genetic variants considered sensi-
tive (Marchand et al. 2004). A number of fitness disadvantages were also observed in
F. het-
eroclitus
resident in polluted estuaries, including poor prey-capture capacity and reduced
salinity range tolerance for egg fertilization (reviewed by Burnett et al. 2007). A recent
study also highlighted a fitness cost of copper resistance in the marine copepod
Tigriopus
japonicus
, as the intrinsic population growth rate of the copper-resistant lineage was sig-
nificantly lower than that of the control (Kwok et al. 2009).
14.5 Toxicant-Altered Gene Expression
The action of environmental stress on an organism can affect the intracellular transfer
of genetic information at sublethal levels. Toxicogenomic approaches using the tools of
molecular biology provide an understanding of a new aspect of the impacts of chemical
contaminants on living organisms. The level of genetic information transfer is assessed by
the measurement of the rate of transcription of those products, which can affect—directly
or indirectly—the processes of detoxification and/or resistance. At present, studies are
performed in controlled experiments in which organisms are exposed to one contaminant
or to a mixture of contaminants in order to evaluate the potential effects of synergy or
antagonism between different molecules. The objective is to identify transcripts whose
level of expression varies significantly between exposed and unexposed organisms. The
information given by the entire sequenced genomes of particular species allows the rec-
ognition of a large number of sequences, and these are accessible in databanks. Moreover,
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